Cylindrical alkaline electrochemical cells commonly employ bobbin type constructions such that a centrally located anode is surrounded by a tubularly shaped cathode. Electrical connection to the anode is generally effected by the use of an elongated metal member, such as a nail, which is forcibly driven through a resilient nonconductive seal body that closes the open end of a cup-shaped container which houses electrochemically active components. One end of the collector protrudes above the seal for electrical connection to an external circuit. The shank of the collector is inserted into the anode where it functions as a current collector. A small portion of the collector resides within the seal body and is in sealing engagement therewith.
Prior to manufacturing a cell, the current collector and seal body are usually preassembled to form a current collector assembly. The seal assembly has a centrally located opening through which the collector protrudes. The outer edge of the seal body and the portion of the seal body which surrounds the centrally located opening are usually reinforced by thickening of the seal body's material. At least one portion of the seal body between the reinforced areas may be made thinner in cross-section than the reinforced areas so that the seal can rupture if the cell's internal pressure exceeds a predetermined limit.
The reinforced portion of the seal which surrounds and defines the centrally located opening is commonly referred to as the "hub". A current collector is inserted through the opening such that an interference fit exists between the seal's hub and the collector whose diameter is greater that the inside diameter of the seal body's opening. The objective is to create an interference fit between the collector and hub such that electrolyte cannot escape from the cell by creeping along the surface of the collector. The extent of the interference must be carefully controlled. If the interference fit results in the creation of tangential tension which exceeds the seal hub's radial hoop strength, the seal will split and allow electrolyte to escape. If the interference fit is insufficient, electrolyte can creep along the surface of the collector and escape from the cell.
U.S. Pat. No. 3,740,271 discloses the use of a clamping ring arranged around the sleeve portion of the plug in order to exert compression on the conductive rod when it is inserted through the sleeve. The conductive rod is inserted into the opening on the exterior side of the plug and then driven through the plug and ring reinforced sleeve on the opposite (i.e. interior) side of the plug.
U.S. Pat. No. 5,008,161 discloses a cap assembly comprising a metallic sleeve which is surrounded on both sides by plastic and located on the interior surface of the seal body in order to compress the plastic against the collector and prevent leakage of electrolyte along the surface of the collector.
U.S. Pat. No. 5,051,323 discloses a conventional seal design in which the collector is inserted through the seal's central opening so that the upstanding wall which surrounds the collector is forced outward against a metal component known as the inner cover. Although the inner cover supports the outer circumference of the seal body, the collector exerts tangential tension against the seal's upstanding wall throughout the life of the collector assembly.
In another example, a commercially available battery discloses a flat metallic ring placed around the outer surface of the seal body. The seal is compressed between the collector and ring as the collector is forced through the seal body's central opening.
The prior art current collector assemblies have the following disadvantages. First, the seal bodies with no support around the hub (i.e. reinforced portion of the seal body through which the collector is inserted) are prone to leakage of electrolyte. This mode of seal failure is referred to as "hub splitting" since the hub splits due to the excessive tangential tension created by the interference fit between the collector and hub. Second, when a compression ring is used on the interior surface of the seal body such that the ring is exposed to the cell's components, the ring may react with one or more of the cell's components and generate gas within the cell. This gassing problem generally exists at an acceptably low level when mercury is used as a corrosion inhibitor in alkaline cells but the gassing becomes excessive when mercury is removed from the cell. The need for battery manufacturers to eliminate mercury precludes the use of a metallic compression ring inside the cell. Third, when the compression ring is located on the exterior of the seal and the collector is inserted in the seal body's opening from the exterior surface of the seal body, the insertion motion tends to drive the seal downward and out of the compression ring. This leads to improperly manufactured current collector assemblies that may allow electrolyte to creep along the surface of the collector when the assembly is incorporated into a cell.
While the known current collector assemblies have been used for many years, there is a need for an improved current collector assembly that simultaneously (1) imparts no tangential tension on the seal body's hub, thereby avoiding hub splitting and (2) provides sufficient radial compressive stress and tangential compressive stress on the seal's hub thereby preventing creepage of electrolyte along the surface of the collector. Preferably, the compression means is located outside the cell's internal environment in order to avoid gassing within the cell and the seal is driven into the compression means during the collector assembly manufacturing process.